1. Field of the Invention
This invention relates to the characteristics of capacitors, and more particularly to methods for reducing or eliminating dielectric absorption error in circuits using capacitors.
2. Description of the Relevant Art
A capacitor comprises two parallel conductive plates separated by a dielectric. The dielectric may be composed of various types of materials. Dielectric absorption refers to the small amount of excess charge absorbed or released by a dielectric after a capacitor has been charged or discharged. Dielectric absorption is a property that every capacitor exhibits to differing degrees. The amount of dielectric absorption for a particular capacitor depends primarily on the type of dielectric material used and the amount of dielectric material in the capacitor. It is desirable to compensate for the effects of dielectric absorption to achieve a more ideal and predictable capacitance.
The effects of dielectric absorption can be seen when a known amount of charge is deposited on a capacitor and the voltage across the capacitor is then observed. The voltage will decay at a constantly decreasing rate, causing the voltage at any given time to be slightly below the level of voltage initially observed across the capacitor. It is believed that this lost charge is absorbed by the dielectric and is, therefore, not apparent in a voltage reading across the capacitor.
The effects of dielectric absorption may be seen by following the following procedure. First a battery is used to charge a large-value tantalum capacitor to a level of 10 volts. Next, after the capacitor is fully charged and has reached a voltage level of 10 volts, the battery is removed. At this point, the voltage across the capacitor should be 10 volts. Now the capacitor may be rapidly discharged by momentarily putting a 100 ohm resistor across it. The resistor is then removed. When the voltage across the capacitor is measured again, the voltage across the capacitor may be observed increasing in voltage, even though the capacitor was previously discharged. The voltage across the capacitor may perhaps reach a volt or so after a few seconds. Dielectric absorption is the cause of the voltage across the capacitor increasing in voltage despite the fact the capacitor was discharged.
Dielectric absorption typically is less than 1% of the total charge applied to a capacitor. Some applications are not substantially affected by this degree of error, but most applications encourage the removal of as much error as possible. Because dielectric absorption varies greatly depending on the type of dielectric used in the capacitor, one way to minimize dielectric absorption is by choosing a dielectric that is less susceptible to this absorption. An air dielectric capacitor, for example, has a very small absorption rate, but is very impracticable in most applications. Air dielectric capacitors are physically much larger (by orders of magnitude) and much more expensive than lower quality capacitors with similar capacitive ratings. Another potential solution is to allow sufficient time between charging periods to allow the current associated with dielectric absorption to drop to an acceptable level. Other methods of partially compensating for dielectric absorption have been used such as providing an "equivalent and opposite impedance" to the dielectric absorption in a capacitive load (U.S. Pat. No. 5,557,242) and using a series of RC circuits to compensate for dielectric absorption (U.S. Pat. No. 5,519,328). However, improved methods are desired for compensating the dielectric absorption error of a capacitor.